As a faster packet transmission system in IMT-2000, a system called HSDPA (High Speed Downlink Packet Access) has been studied for purposes of fast downlink peak transmission rate, low transmission delay, and high throughput, for example. As techniques for supporting HSDPA, a transmission system called AMC (Adaptive Modulation and coding) is disclosed in 3GPP (3rd Generation Partnership Project) TR25.848 “Physical layer aspects of UTRA High Speed Downlink Packet Access”.
The AMC technique is to vary the modulation scheme and error correcting coding rate adaptively at high speed corresponding to variation in channel quality. In the AMC technique, as the channel quality is higher, a faster modulation scheme is used, thereby the error correcting coding rate is increased, and thus the transmission rate is increased. Specifically, a mobile station (or base station) measures downlink propagation conditions whenever necessary for each mobile station, and based on the measurement result, the base station determines a mobile station to transmit information and an optimal transmission rate to transmit the information. Regarding the transmission rate, considered are modulation schemes (for example, QPSK (Quaternary Phase Shift Keying) and 16QAM (Quaternary Amplitude Modulation)), coding rate (for example, turbo coding with a coding rate of ⅓ while performing puncture or repetition to vary the coding rate). Examples used as channel quality information include CIR (Carrier to Interference Ratio), SIR (Signal to Interference Ratio), TFRC (Transport Format and Resource Combination), and transmission power of a dedicated channel (for example, DPCH (Dedicated physical Channel)).
For example, in an application example in determining a transmission rate in the AMC technique, a base station determines a modulation scheme and coding rate (MCS: Modulation and Coding scheme), i.e., items associated with the transmission rate, based on CIR reported from a mobile station or the transmission rate of a dedicated channel.
However, in such determination of a transmission rate, there are following problems. That is, even when a transmission rate is determined based on information of instantaneous moment, the reliability of the information deteriorates due to effects of delay in assignment, moving speed of a mobile station, and/or accuracy in measuring channel quality in the mobile station or base station. For example, when the moving speed of a mobile station is low (the Doppler frequency is small), propagation environments do no vary greatly. However, when the moving speed is high (the Doppler frequency is large), propagation environments vary greatly, and the reliability of the information deteriorates. As a result, it is not possible to optimize the assignment of transmission rate, and there arise a possibility of having a significant effect on the throughput.
In addition, such an issue is not limited to HSDPA as described above, and arises widely in systems for controlling the transmission rate. In other words, although the spreading factor is fixed in HSDPA, since determination of the transmission rate is also related to the spreading factor and transmission power in addition to the modulation scheme and coding rate, it is not possible to optimize the determination of transmission rate when reliabilities of the above-mentioned pieces of information deteriorate, and there arises the possibility of having a significant effect on performance of the transmission system.